Police said a rocket was fired at the US eagle emblem at the front of the building.

"A self-propelled explosive was fired at 0558 (0358 GMT) from the surrounding area, causing minor damage to the front windows and the roof," a statement from the Greek public order ministry said.

The rocket landed in a toilet on the third floor of the building, which also houses Ambassador Charles Ries's office.

"I am treating this as a very serious attack," Mr Ries said. "The embassy was attacked in a senseless act of violence."

Mr Ries said the reliability of any claims of responsibility would have to be assessed.

Television pictures showed a mass of emergency vehicles and stationary traffic outside the embassy as the area was sealed off.

Left-wing groups

The US embassy is one of the most fortified and tightly guarded buildings in the region and is frequently the target of protests in a country brimming with potent anti-Americanism, says the BBC's Malcolm Brabant in Athens.

In February 1996, it suffered minor damage when unknown attackers - thought to be leftist radicals - fired a rocket at it.

In the past, the far-left November 17 group - now disbanded and whose leaders were jailed in 2003 - attacked Greek, US and other foreign targets, killing more than 20 people.

Revolutionary Struggle - a group which emerged after November 17 disintegrated - is regarded by security experts as Greece's most active terrorist group, our correspondent says.

It has broadly the same left-wing, anarchic anti-capitalist agenda as November 17 and vigorously opposes America's intervention in Iraq and Afghanistan, as well as the economic and social policies of the current conservative Greek government.